If you run backup of these vms from inside a guest OS, then missed vms could have been offline at that time; if you back up these vms at a hypervisor level (agentless backup), then we need to investigate this issue.

Yes, you are right, a device is not deleted from a backup plan when it is not accessible, however, a backup plan cannot be performed for this specific device: it will be performed next time when it is accessible.

At any way, I suggest that you open a case with Acronis Support Team who will analyze why a backup plan has not been performed on VMs in your situation.

You have not mentioned this above. This issue definitely should be investigated by Acronis support engineers since it needs to be reproduced. We have not met such issues before.

When you have open a case with Acronis Support Team, please share with them Acronis System Report collected from a machine where Acronis Management Server is installed and your backup plans (please export them to files).